Aircraft are generally equipped with fuel tanks located in the wings and centre wing box. These main tanks are typically permanently fitted to the aircraft and are of a rigid construction. The main tanks typically comprise the wing and centre wing box tanks. A multiplicity of fuel gauges are fitted to the main tanks such that the quantity of remaining fuel in the main tanks can be accurately determined, which is important in ensuring safe flight operations. A fuel indicator provides a measurement of the fuel remaining in the tanks that is typically accurate to 1%. The system of fuel gauges utilised ensures that a high integrity system is achieved, having a mean time to failure of greater than 1×109 hours.
It is possible to fit auxiliary fuel tanks to aircraft to increase the fuel capacity and therefore increase the range of the aircraft. Such auxiliary fuel tanks may be fitted in the fuselage of the aircraft under the flooring, and are often made of flexible material, such as polyurethane, to enable them to be fitted around the structure of the aircraft without the need to dismantle that structure, thereby allowing fitment and removal as required. Auxiliary fuel tanks typically have a capacity of between about 5% and 50% of the main tank capacity.
When operating with auxiliary tanks it is highly desirable to provide a means for indicating the total fuel available, that is, the sum of the fuel in both the main tanks and the auxiliary tanks. This could be achieved by providing fuel gauges on the auxiliary tanks, but the fitment of fuel gauges to the auxiliary tanks would be costly and would add weight to the tanks. Furthermore, such auxiliary gauges might need to be connected to the fuel monitoring system of the aircraft so that a correct indication of the fuel remaining could be displayed, which would further complicate their fitment. Even if gauges were fitted to flexible auxiliary tanks it would be difficult to obtain an accurate measure of the fuel quantity in such tanks due to the flexible nature of the tank and the complex shapes of the tanks.